Mrs Clinton did not suffer a stroke or neurological damage, the doctors said, in a statement that revealed the location of the clot - in the vein in the space between the brain and the skull, behind the right ear.

The former first lady is being treated with blood thinners to help dissolve the clot and will be allowed home once the medication dose has been established.

Mrs Clinton, 65, was making excellent progress and was in good spirits, Dr Lisa Bardack of the Mt Kisco Medical Group and Dr Gigi El-Bayoumi of George Washington University said.

Mrs Clinton, who was spending a second day in hospital, developed the clot after suffering concussion earlier this month when she fainted, fell and struck her head at home while battling a stomach virus. She has not been seen publicly since December 7.

Phillipe Reines, her spokesman, said her doctors discovered the clot on Sunday while performing a follow-up exam on the concussion. She was admitted to New York-Presbyterian Hospital.

Mrs Clinton's complication "certainly isn't the most common thing to happen after a concussion" and is one of the few types of blood clots in the skull or head that are treated with blood thinners, said Dr Larry Goldstein, a neurologist who is director of Duke University's stroke centre. The area where her clot developed is "a drainage channel, the equivalent of a big vein inside the skull - it's how the blood gets back to the heart", Dr Goldstein said, adding that blood thinners are usually enough to treat the clot and it should have no long-term consequences if her doctors are saying she has suffered no neurological damage, he said.

Mrs Clinton had planned to step down as secretary of state at the beginning of President Barack Obama's second term. Whether she will return to work before she resigns remains a question.

Democrats are privately, if not publicly, speculating on how her illness might affect a decision about running for president in 2016. Her age - and thereby health - would likely be a factor under consideration, given that Mrs Clinton would be 69 when sworn in, if she were elected in 2016.

After decades in politics, Mrs Clinton says she plans to spend the next year resting. She has long insisted she had no intention of mounting a second campaign for the White House four years from now. But the door is not entirely closed and she would almost certainly emerge as the Democrat to beat if she decided to give in to calls by supporters and run again.